The present invention relates to a method for rapid synchronization of a point to multipoint modem system and, in particular, to a method for increasing the efficiency of processing of a received signal by storing the carrier frequency offset and equalizer taps between processing of burst transmissions.
Many different types of communication systems feature a master modem, at a base station, which then controls the transmissions of a plurality of slave modems, at terminal stations. For example, such a communication system is often used for wireless communication according to a TDM (time division multiplexing) protocol, an FDD (frequency division duplex) protocol and a TDMA (time division multiple access) protocol. Transmission from the base station to the terminal stations for the downstream transmissions is performed at one carrier frequency, and transmissions from the terminal stations to the base station, or upstream transmissions, are performed at another frequency, such that the downstream and upstream transmissions do not interfere with each other. For upstream transmissions from the terminal stations to the base station, communication follows the TDMA protocol. Downstream transmissions, however, are continuous and are performed according to the TDM protocol, for example. The base station determines the timing of transmissions by the terminal stations. In order for communication to occur, a transmitted signal must be accurately processed by the receiver for accurate recovery of the information contained within the signal.
For example, radio frequency signals are subject to distortion such as amplitude and phase distortion, carrier frequency offset and phase noise. Amplitude and phase distortion, which cause time dispersion, are known as channel response. The transmission frame may include synchronization fields, which are required for correct processing of the frame to overcome the above-mentioned distortions, yet which must be minimized in order to maximize the available bandwidth. In the upstream direction, due to the bursty nature of the transmission, every burst must include synchronization fields. For example, these fields may appear at the beginning of the burst, and then they form the header. When bursts are relatively short, the synchronization overhead must be decreased as much as possible, such that the synchronization fields must be as short as is practical.
One typical approach to compensate for channel response (amplitude and phase distortion and time dispersion of the signal) in a receiver is an equalizer. The equalizer includes a plurality of xe2x80x9ctapsxe2x80x9d, each of which has an associated equalizer coefficient, with a delay separating each tap. The equalizer coefficients may be calculated using an iterative approach, where the coefficients are adjusted until the coefficients converge acccording to some algorithm, such as LMS (least mean squares) (see for example chapter 6 of Adaptive Signal Processing, by. B. Widrow and S. D. Stearns, Prentice-Hall, Inc., Englewood Cliffs, N.J., USA, 1985). Since the channel response is typically time variant for wireless communication, equalizers are usually adaptive, such that the equalizer coefficients are varied over time to be able to track the changes in the channel response. The rate at which the equalizer coefficients are adjusted depends upon the rate at which the channel response varies over time. Typically for background art equalizers for wireless communication according to TDMA, the equalizer coefficients are adjusted for each burst alone, without reference to any previous burst, which requires longer synchronization fields and lengthier calculations.
One example of a system which attempts to increase the speed for signal processing is disclosed in U.S. Pat. No. 4,847,880. The disclosed system stores certain equalizer coefficients for one initial training session, and then uses these stored coefficients for all subsequent equalizations of the signal. The equalizer coefficients are not updated at any later time. This approach is only suitable for a channel which is not varying. Thus, the storage of these coefficients is used to increase the efficiency of processing, but is inoperative for a time varying channel, such as for wireless communication.
However, for certain types of applications, in particular fixed systems in which both the base station and the terminal stations are stationary, various values calculated during the signal processing change relatively slowly between consecutive bursts, but these values are still time variant. For example, the equalizer coefficients typically do not change significantly between bursts. Therefore, using the values obtained from the calculation of equalizer coefficients for a previous burst transmission as the basis for the calculation for a subsequent consecutive burst from the same terminal station, rather than starting from a random or otherwise unrelated value, could significantly improve the efficiency of the equalizer coefficient adaptation. Unfortunately, no currently available system takes advantage of those receiver parameters which change relatively slowly.
There is thus a widely recognized need for, and it would be highly advantageous to have, a system and a method for storing certain relatively slowly changing receiver parameters between transmissions for signal processing, such as the equalizer coefficients and the carrier frequency offset, in order to increase the efficiency of signal processing, and decrease the length of synchronization fields, thereby maximizing available bandwidth.
According to the present invention, there is provided a system for communication between a base station and at least one terminal station, the base station receiving a plurality of sequentially transmitted bursts from the at least one terminal station according to a TDMA (time division multiple access) protocol, each burst featuring a synchronization portion, each of the plurality of sequentially transmitted bursts being denoted Si wherein i is an integer, the plurality of sequentially transmitted bursts including at least a burst Sm and a burst Sn being sent from one terminal station, wherein m and n are each integers and m less than n, the system comprising: a base station transceiver comprising: (i) a receiver unit for receiving the plurality of bursts Si, including at least the burst Sm and the burst Sn, and for producing each of a plurality of analog signals from each of the plurality of bursts Si; (ii) an analog-to-digital converter for converting each analog signal to a sampled digital signal, by sampling each analog signal according to a sampling timing; (iii) an adaptive equalizer for equalizing each sampled digital signal according to each of a plurality of sets of equalizer coefficients to produce an equalized signal, such that a sampled digital signal corresponding to a particular burst Si is equalized according to a particular set of equalizer coefficients; and (iv) an equalizer controller for determining each set of coefficients by processing a portion of an equalized signal corresponding to the synchronization portion of a burst Si according to an initialized coefficient adjustment procedure; such that for processing a portion of an equalized signal corresponding to the synchronization portion of the burst Sn, the initialized coefficient adjustment procedure is initialized by adapting a set of coefficients at least partially determined by processing a portion of an equalized signal corresponding to the synchronization portion of the burst Sm.
Preferably, n=m+1 and the equalized signal corresponding to the burst Sn is equalized according to the set of coefficients produced by processing the portion of the equalized signal corresponding to the synchronization portion of the burst Sn.
Alternatively and preferably, the plurality of bursts Si includes a burst Sq, q being an integer and q greater than n, and an equalized signal corresponding to the burst Sq is equalized according to the set of coefficients produced by processing the portion of the equalized signal corresponding to the synchronization portion of the burst Sn.
According to a preferred embodiment of the present invention, there is provided an equalized signal corresponding to the burst Sn is an Sn-corresponding equalized signal, and the base station transceiver further comprises: (v) an automatic carrier frequency control for rotating a phase of the Sn-corresponding equalized signal according to a phase offset of the Sn-corresponding equalized signal to produce a rotated signal, the phase offset being determined according to a carrier frequency offset of the Sn-corresponding equalized signal, the automatic carrier frequency control featuring: a loop filter for adapting the carrier frequency offset of the Sn-corresponding equalized signal from a carrier frequency offset determined at least partially from an equalized signal corresponding to the burst Sm.
Preferably, an initial carrier phase is assumed at a start of processing the portion of each equalized signal corresponding to the synchronization portion of each burst Si, the automatic carrier frequency control further featuring: (2) a phase initializer for determining a phase error according to the synchronization portion of the Sn-corresponding equalized signal and for correcting the initial carrier phase according to the phase error to determine a correct phase, such that a set of equalizer coefficients being processed according to the synchronization portion corresponding to the burst Sn is also processed according to the correct phase by the initialized coefficient adjustment procedure.
Preferably, each burst features a header and a traffic portion, and the header is the synchronization portion, the header being processed a first time for determining the correct phase and the header being processed a second time for processing the set of equalizer coefficients.
Also preferably, the base station transceiver further comprises: (v) at least one digital gain for correcting a gain of the second sampled digital signal.
According to other preferred embodiments of the present invention, the base station transceiver further comprises: (v) a transmitter for receiving a digital input, for converting the digital input to an analog signal and for transmitting the analog signal to the terminal station, the transmitter featuring a digital-to-analog converter for converting the digital input to produce the analog signal according to a base station transmission clock; wherein the sampling timing of the analog-to-digital converter of the base station transceiver is determined directly according to the base station transmission clock, and wherein the system further comprises: (b) a terminal station transceiver, the terminal station transceiver featuring: (i) a receiver unit for receiving the analog signal from the base station; (ii) a symbol timing recovery unit for recovering the base station transmission clock from the analog signal to form a recovered clock; (iii) an analog-to-digital processing unit for processing the analog signal to a processed digital signal according to the recovered clock; and (iv) a transmitter for receiving a digital input, for converting the digital input to an analog signal and for transmitting the analog signal to the base station, the transmitter featuring a digital-to-analog converter for converting the digital input to produce the analog signal according to the recovered clock.
Preferably, the analog-to-digital processing unit features an analog-to-digital converter for converting the analog signal to a sampled digital signal by sampling the analog signal according to a terminal station reception clock, the terminal station reception clock being determined directly according to the recovered clock.
Alternatively and preferably, the analog-to-digital processing unit features: (1) an analog-to-digital converter for converting the analog signal to a sampled digital signal by sampling the analog signal according to a terminal station reception clock, the terminal station reception clock being determined directly according to a free-running oscillator; and (2) at least one interpolating receive filter for filtering the sampled digital signal directly according to the recovered clock; and wherein the transmitter features at least one interpolating filter for filtering the digital input directly according to the recovered clock.
More preferably, the symbol timing recovery unit features: (A) a loop filter for determining a timing frequency offset of the sampled digital signal; and (B) an integrator for determining a timing phase shift according to the timing frequency offset; such that the at least one interpolating receive filter performs an interpolation of the sampled digital signal also according to the timing phase shift.
Most preferably, the at least one interpolating receive filter performs the interpolation of the sampled digital signal by precomputing a plurality of interpolations according to a plurality of possible phase shifts, and then selecting one of the plurality of interpolations for interpolating the sampled digital signal according to an actual phase shift of the sampled digital signal.
According to another embodiment of the present invention, there is provided a system for communication between a base station and at least one terminal station, the base station receiving a plurality of sequentially transmitted bursts from the at least one terminal station according to a TDMA (time division multiple access) protocol, each burst featuring a synchronization portion, each of the plurality of sequentially transmitted bursts being denoted Si wherein i is an integer, the plurality of sequentially transmitted bursts including at least a burst Sm and a burst Sn being sent from one terminal station, wherein m and n are each integers and m less than n, the system comprising: a base station transceiver comprising: (i) a receiver unit for receiving the plurality of bursts Si, including at least the burst Sm and the burst Sn, and for producing each of a plurality of analog signals from each of the plurality of bursts Si; (ii) a digital receiver front-end unit for converting each of the plurality of analog signals to each of a plurality of processed digital signals, a processed signal corresponding to the burst Sn being an Sn-corresponding processed signal; and (iii) an automatic carrier frequency control for rotating a phase of the Sn-corresponding processed signal according to a phase offset of the Sn-corresponding processed signal to produce a rotated signal, the phase offset being determined according to a carrier frequency offset of the Sn-corresponding processed signal, the automatic carrier frequency control featuring: a loop filter for adapting the carrier frequency offset of the Sn-corresponding processed signal from a carrier frequency offset determined at least partially from a processed signal corresponding to the burst Sm.
Preferably, an initial carrier phase is assumed at a start of processing the portion of each equalized signal corresponding to the synchronization portion of each burst Si, the automatic carrier frequency control further featuring: (2) a phase initializer for determining a phase error according to the synchronization portion of the Sn-corresponding equalized signal and for correcting the initial carrier phase according to the phase error to determine a correct phase, such that a set of equalizer coefficients being processed according to the synchronization portion corresponding to the burst Sn is also processed according to the correct phase by the initialized coefficient adjustment procedure.
According to yet another embodiment of the present invention, there is provided a method for communication between a base station and at least one terminal station, the base station receiving a plurality of sequentially transmitted bursts from the at least one terminal station according to a TDMA (time division multiple access) protocol, each burst featuring a synchronization portion, each of the plurality of sequentially transmitted bursts being denoted Si wherein i is an integer, the plurality of sequentially transmitted bursts including at least a burst Sm and a burst Sn being sent from one terminal station, wherein m and n are each integers and m less than n, the base station featuring a base station transceiver, the steps of the method being performed by the base station transceiver, the method comprising the steps of: (a) receiving the plurality of bursts Si, including at least the burst Sm and the burst Sn; (b) producing each of a plurality of analog signals from each of the plurality of bursts Si; (c) converting each analog signal to a sampled digital signal, by sampling each analog signal according to a sampling timing; and (d) processing each sampled digital signal according to at least one parameter to form a digital output, the at least one parameter being an adapted parameter, such that for processing a sampled digital signal corresponding to burst Sn, the adapted parameter is adapted according to another parameter at least partially determined by processing a sampled digital signal corresponding to the burst Sm.
Preferably, the at least one parameter is a set of equalizer coefficients, and the step of processing the sampled digital signal includes the steps of: (i) equalizing the sampled digital signal according to the set of equalizer coefficients; and (ii) determining each set of coefficients by processing a portion of an equalized signal corresponding to the synchronization portion of a burst Si according to an initialized coefficient adjustment procedure, such that for processing a portion of an equalized signal corresponding to the synchronization portion of the burst Sn, the initialized coefficient adjustment procedure is initialized by adapting a set of coefficients at least partially determined by processing a portion of an equalized signal corresponding to the synchronization portion of the burst Sm.
More preferably, n=m+1 and the equalized signal corresponding to the burst Sn is equalized according to the set of coefficients produced by processing the portion of the equalized signal corresponding to the synchronization portion of the burst Sn.
Alternatively and more preferably, the plurality of bursts Si includes a burst Sq, q being an integer and q greater than n, and an equalized signal corresponding to the burst Sq is equalized according to the set of coefficients produced by processing the portion of the equalized signal corresponding to the synchronization portion of the burst Sn.
According to still other preferred embodiments of the present invention, an equalized signal corresponding to the burst Sn is an Sn-corresponding equalized signal, and the step of processing the sampled digital signal further comprises the steps of: (iii) determining a carrier frequency offset of the Sn-corresponding equalized signal by adapting the carrier frequency offset of the Sn-corresponding equalized signal from a carrier frequency offset determined at least partially from an equalized signal corresponding to the burst Sm; (iv) determining a carrier phase offset according to the carrier frequency offset of the Sn-corresponding equalized signal; and (v) rotating a phase of the Sn-corresponding equalized signal according to the carrier phase offset of the Sn-corresponding equalized signal to produce a rotated signal.
According to still another embodiment of the present invention, there is provided a method for communication between a base station and at least one terminal station, the base station receiving a plurality of sequentially transmitted bursts from the at least one terminal station according to a TDMA (time division multiple access) protocol, each burst featuring a synchronization portion, each of the plurality of sequentially transmitted bursts being denoted Si wherein i is an integer, the plurality of sequentially transmitted bursts including at least a burst Sm and a burst Sn being sent from one terminal station, wherein m and n are each integers and m less than n, the base station featuring a base station transceiver, the base station transceiver featuring a receiver unit, an analog-to-digital converter, an adaptive equalizer and an equalizer controller, the method comprising the steps of: (a) receiving the plurality of bursts Si by the receiver unit, including at least the burst Sm and the burst Sn; (b) producing each of a plurality of analog signals from each of the plurality of bursts Si; (c) converting each analog signal to a sampled digital signal by the analog-to-digital converter, by sampling each analog signal according to a sampling timing; (d) equalizing the sampled digital signal according to the set of equalizer coefficients by the equalizer; and (e) determining each set of coefficients by the equalizer controller by processing a portion of an equalized signal corresponding to the synchronization portion of a burst Si according to an initialized coefficient adjustment procedure, such that for processing a portion of an equalized signal corresponding to the synchronization portion of the burst Sn, the initialized coefficient adjustment procedure is initialized by adapting a set of coefficients at least partially determined by processing a portion of an equalized signal corresponding to the synchronization portion of the burst Sm.
According to yet other preferred embodiments of the present invention, the base station transceiver features an automatic carrier frequency control and an equalized signal corresponding to the burst Sn is an Sn-corresponding equalized signal, the method further comprising the steps of: (f) determining a carrier frequency offset of the Sn-corresponding equalized signal by the automatic carrier frequency control by adapting the carrier frequency offset of the Sn-corresponding equalized signal from a carrier frequency offset determined at least partially from an equalized signal corresponding to the burst Sm; (g) determining a carrier phase offset according to the carrier frequency offset of the Sn-corresponding equalized signal; and (h) rotating a phase of the Sn-corresponding equalized signal according to the carrier phase offset of the Sn-corresponding equalized signal to produce a rotated signal.
Preferably, each burst features a header and a traffic portion, and the header is the synchronization portion, the method further comprising the steps of: (i) assuming an initial carrier phase at a start of processing a portion of each equalized signal corresponding to the header of a burst Si; (j) determining a phase error according to the portion of the equalized signal corresponding to the header of a burst Si; (k) correcting the initial carrier phase according to the phase error to form a correct phase; and (l) adapting each set of equalizer coefficients according to the correct phase, such that a set of equalizer coefficients employed to equalize a digital sampled signal corresponding to the burst Sn is also processed according to the correct phase by the initialized coefficient adjustment procedure.
More preferably, the base station transceiver additionally features a transmitter and the terminal station features a terminal station transceiver, the method further comprising the steps of: (m) receiving a digital input by the transmitter of the base station transceiver; (n) converting the digital input to an analog signal according to a base station transmission clock, such that the sampling timing of the base station transceiver for performing the step of converting each analog signal to a sampled digital signal is determined directly according to the base station transmission clock; (o) transmitting the analog signal to the terminal station; (p) receiving the analog signal from the base station by the terminal station transceiver; (q) recovering the base station transmission clock from the analog signal to form a recovered clock; (r) processing the analog signal to a processed digital signal according to the recovered clock, the processed digital signal being output from the terminal station transceiver; (s) receiving a digital input into the terminal station transceiver; (t) converting the digital input to an analog signal according to the recovered clock, such that the sampling timing of the base station transceiver has a substantially identical frequency as the recovered clock; and (u) transmitting the analog signal to the base station.
Most preferably, the step of processing the analog signal to the processed digital signal by the terminal station transceiver further comprises the step of: (i) converting the analog signal to a sampled digital signal by sampling the analog signal according to a terminal station reception clock, the terminal station reception clock being determined directly according to the recovered clock.
Alternatively and most preferably, the step of processing the analog signal to the processed digital signal by the terminal station transceiver further comprises the steps of: (i) converting the analog signal to a sampled digital signal by sampling the analog signal according to a terminal station reception clock, the terminal station reception clock being determined directly according to a free-running oscillator; and (ii) interpolating the sampled digital signal directly according to the recovered clock; and wherein the step of converting the digital input to an analog signal is performed by interpolating the digital input according to the recovered clock. Preferably, the step of interpolating the sampled digital signal further comprises the steps of: (1) determining a symbol timing frequency offset of the sampled digital signal; (2) determining a symbol timing phase shift according to the symbol timing frequency offset; and (3) performing an interpolation of the sampled digital signal also according to the symbol timing phase shift; and wherein the step of interpolating the digital input further comprises the step of adjusting a phase of the digital input according to the symbol timing phase shift.
Preferably, the step of performing the interpolation of the sampled digital signal further comprises the steps of: (A) precomputing a plurality of interpolations according to a plurality of possible symbol timing phase shifts; and (B) selecting one of the plurality of interpolations for interpolating the sampled digital signal according to an actual symbol timing phase shift of the sampled digital signal.
More preferably, the step of adjusting a phase of the digital input according to the phase shift further comprises the steps of: (1) precomputing a plurality of interpolations according to a plurality of possible symbol timing phase shifts; and (2) selecting one of the plurality of interpolations for interpolating the digital input according to an actual symbol timing phase shift of the digital input.
Hereinafter, a signal which corresponds to a particular burst Si(i being an integer) is defined as having been produced by processing at least a portion of burst Si, for example by sampling an analog signal obtained from the received burst Si to produce a xe2x80x9ccorrespondingxe2x80x9d digital sampled signal.